1. Field of the Invention
The present invention relates to planar or xe2x80x9cintegratedxe2x80x9d optical waveguides, and particularly to lithographically-formed single-mode waveguides employing organic and polymeric materials.
2. Technical Background
Multilayer optical waveguiding structures are used to build integrated optical circuits that-route and control optical signals in a optical fiber communication system. In optical communication systems, messages are transmitted at infrared optical frequencies by carrier waves that are generated using sources such as lasers and light-emitting diodes. There is interest in these optical communication systems because they offer several advantages over electronic communications systems using copper wires or coaxial cable. They have a greatly increased number of channels of communication, as well as the ability to transmit messages at much higher speeds than electronic systems.
This invention is concerned with the formation of light-transmissive optical waveguide devices. The operation of an optical waveguide is based on the fact that when a core medium which is transparent to light is surrounded or otherwise bounded by another cladding medium having a lower refractive index, light introduced along the core medium""s axis is highly reflected at the boundary with the surrounding cladding medium, thus producing a light-guiding effect.
It is possible to produce polymeric optical waveguides and other optical devices which transport optical signals via optical circuitry or optical fiber networks. One method used to form an optical waveguide device involves the application of standard photolithographic processes. Photopolymers are of particular interest for optical applications because they can be patterned by photolithographic techniques which are well known in the art. Photopolymers also offer opportunities for simpler, more cost-effective manufacturing processes. Lithographic processes are used to define a pattern in a light-sensitive, photopolymer-containing layer deposited on a substrate. This layer may itself consist of several layers composed of the same or different polymeric materials having dissimilar refractive indices, to form a core, overcladding, and under cladding layers or structures.
Among the many known photopolymers, acrylate materials have been identified as suitable for optical waveguides because of their optical clarity, low birefringence, and ready availability of a wide range of monomers.
Planar polymer waveguides typically comprise layers of low loss optical materials of precise indices of refraction. Both step index and gradient index waveguide structures are known in the art. For planar polymer and glass waveguides, step index structures are most easily achieved through successive coating of materials with differing refractive indices. Typically, a core has a refractive index which is 0.5% to 2% higher than its overcladding. The magnitude of this refractive index difference (xcex94n) is set to optimize the performance of the planar waveguides, or to match light modes when the transition is made from the planar device to an optical fiber.
In practice, most planar waveguide structures have a configuration wherein a buffer layer is applied to a silicon substrate, then an underclad is applied to the buffer, followed by application and patterning of a core layer, and followed finally by application of an overclad. In some instances, the buffer layer can serve as the under clad.
If these multiple layers are not optimized, several problems can occur. These include high optical loss due to absorption of light by the substrate; high polarization dependent loss (PDL); if heating is performed (for tuning or switching), the increase of temperature alters the index of refraction in such a way as to push light out at least partially out of the core where is can interact with the cladding and/or the substrate to produce a variety of unwanted interactions which can, for example, lead to loss and PDL; and if the waveguide incorporates a grating, secondary reflections or an unwanted broadening of the wavelength of the reflected signal can be observed.
The invention provides a single-mode optical waveguide fabricated on a substrate, the substrate defining a surface, the single-mode optical waveguide comprising a polymeric, buffer layer is disposed on the surface of the substrate, the polymeric, buffer layer defining a surface and having an index of refraction nb. A patterned, light-transmissive core layer disposed directly on the surface of the buffer layer, the patterned, light-transmissive core layer defining a top surface and a pair of side walls, the patterned, light-transmissive core layer having an index of refraction nc. An overcladding layer is disposed on the top surface of the core, the pair of side walls of the core, and the buffer layer, the overcladding layer having an index of refraction no, such that nb less than no less than nc and xcex94n=ncxe2x88x92no, wherein the value of xcex94n produces a single-mode waveguide at optical communication wavelengths.
The invention also provides a method for forming a single-mode optical waveguide on a substrate, the substrate defining a surface, the method comprising the steps of depositing a polymeric, buffer layer onto the surface of the substrate, the polymeric, buffer layer defining a surface and having an index of refraction nb. One then deposits a patterned, light-transmissive core layer directly onto the surface of the polymeric, buffer layer without any intermediate layers, the patterned, light-transmissive core layer defining a core having a top surface and a pair of side walls, the patterned, light-transmissive core layer having an index of refraction nc. One then deposits an overcladding layer onto the top surface of the patterned, light-transmissive core layer, the side walls of the patterned, light-transmissive core layer, and a portion of the polymeric, buffer layer, the overcladding layer having an index of refraction no, such that nb less than no less than nc and xcex94n ncxe2x88x92no.
The invention further provides a method for forming a single-mode optical waveguide on a substrate, the substrate defining a surface, the method comprising the steps of depositing a buffer layer onto the surface of the substrate, the buffer layer being fabricated from a polymeric, material having an index of refraction nb, the buffer layer defining a surface. One then deposits a core layer directly onto the surface of the buffer layer without any intermediate layers, the core layer being fabricated from a light-transmissive material having an index of refraction nc. One then patterns the core layer to define a core with a top surface and a pair of side walls and to expose portions of the buffer layer. One then deposits an overcladding layer onto the top surface of the core layer, the side walls of the core layer, and the exposed portions of the buffer layer, the overcladding layer having an index of refraction no, such that nb less than no less than nc and xcex94n=ncxe2x88x92no, wherein the value of xcex94n produces a single-mode waveguide at optical communication wavelengths.
The invention still further provides a method for forming an optical waveguide on a substrate, the substrate defining a surface, the method comprising the steps of depositing a polymeric, buffer layer onto the surface of the substrate, the polymeric, buffer layer defining a surface and having an index of refraction nb. One then deposits a photosensitive core layer directly onto the surface of the polymeric, buffer layer without any intermediate layers, the photosensitive core layer defining a top surface, the photosensitive core layer having an index of refraction nc. One then imagewise exposes the light-transmissive core layer to actinic radiation and develops the photosensitive core layer to remove non-image areas of the photosensitive core layer and not remove image areas of the photosensitive core layer, thus forming a patterned, light-transmissive optical waveguide core having a pair of side walls on the polymeric, buffer layer, and partially revealing an exposed portion of the polymeric, buffer layer; and depositing an overcladding layer onto the top surface of the patterned, light-transmissive optical waveguide core, onto the pair of side walls of the patterned, light-transmissive optical waveguide core, and onto the exposed portion of the polymeric, buffer layer, the overcladding layer having an index of refraction n6, such that nb less than no less than nc and xcex94n=ncxe2x88x92no.
The present invention involves asymmetrically cladding the core to solve the above problems. Typically, a cladding is formed uniformly around the core. Adding a buffer of index lower than the clad underneath the under clad solves the problem of loss due to light absorption by the substrate as mentioned above. According to this invention, if the underclad is removed, then each of the above problems is solved. The use of a buffer that has a refractive index much lower than that of the core has several benefits. The buffer keeps the tail of the core mode from extending into the substrate, thus keeping the light from leaking into the substrate. The buffer keeps the light from leaking into the substrate, thus eliminating the main reason for polarization dependent loss (PDL) where TM-polarized light can incur significantly higher loss than TE-polarized light. The buffer has a low enough refractive index to keep the light from leaking into the substrate even when heating is performed. When an under clad is not used and a substantially non-photolocking buffer is used, the formation of a secondary waveguide under the core waveguide is avoided when the core is patterned by exposure to radiation. If the waveguide incorporates a grating and an under clad is not used, secondary reflections due to a guide in the under cladding are avoided. If the waveguide incorporates a grating and an under clad is not used, the buffer photolocks just enough to have a grating imprinted in it when the grating is imprinted holographically in the waveguide and allows one to avoid loss of light by coupling to cladding modes. In this case the term holographically imprinted means a grating that is produced in the volume of the material which comprises the waveguide by a periodic modulation in the index of refraction. Such a grating may be contrasted with a surface-relief grating, which is produced by a periodic variation in the topography of the surface of either the core or cladding of the waveguide. In both cases the effect produces a periodic variation in the effective index of refraction along the propagation direction of light within the waveguide.
Additionally, due to the height of the core, the overclad typically has a bump on it that can be quite large. This can occur in polymer waveguides in which polymers must be spin cast from a solvent solution due to their high molecular weight and viscosity. It can also occur in silica waveguides in which chemical vapor deposition of the overclad applies a uniform layer on top of the core. In addition, reactive-ion etching of polymer or glass waveguide cores can result in high propagation losses due to scattering of light caused by rough side walls. Waveguides can be made using photopolymerizable optical materials which can be coated and cured on a substrate. Typically, the materials include mixtures of monomeric and oligomeric components which are blended to provide the correct index of refraction. Mixtures are blended to provide a an between core and clad, of typically 0.5 to 2 percent. In the photolithography of these curing mixtures, typically a guiding region having an index gradient instead of a step index can be formed in the under clad layer. Also, a region can form at the side and the top of the core in which an index gradient is found instead of a step index. The formation of the gradient index in the region surrounding the core is due to migration of dissimilar chemical components, particularly a monomer component moving from the core layer into the cladding layers. In the region directly under the core, the monomers can further react during the formation of the core forming an unwanted guiding region within the under cladding layer. When the underclad region is of about the same thickness as the core, a guiding layer can be formed that penetrates the full thickness of the under clad. In extreme cases it can be as intensely guiding as the core itself and allows light to reach the substrate surface. Since the substrates of this invention are absorbing at optical wavelengths of importance to telecommunications, any portion of the propagating light that reaches the substrate is subject to absorption. Absorption of light by the substrate leads to a severe undesirable polarization-dependent loss of optical power from the propagating signal. Attempts have been made in the art to resolve these issues. One potential solution uses a thick under cladding layer to isolate the core from the substrate to prevent this undesirable result. This eliminates the problem to the desired degree, however, it requires the use of an impracticably thick under cladding. Another solution includes using a buffer region with an index which is 2% or more lower than the core, wherein the buffer region is below the underclad. Even if monomer diffusion occurs deeply through the under clad and slightly into the buffer, the guiding in the buffer will be greatly suppressed, eliminating most light absorption by the silicon. However, the under clad can still guide light and multimode waveguides with residual polarization effects can still result.
One method of lithographically forming optical elements uses an acrylic photoactive composition which is capable of forming a waveguide material upon polymerization. However, this method requires one to utilize polymers with as high a glass transition temperature as possible in order to provide for the greatest operating temperatures. Another method produces waveguides using light-polymerizable compositions such as acrylics having a Tg of at least 100xc2x0 C. The foregoing waveguides suffer from undesirably high optical loss.
It would be desirable to produce optical devices from polymeric materials which have low absorption and scattering loss at application wavelengths, have precisely controllable refractive indexes for mode and numerical aperture control. Precise refractive index control allows control of mode and numeric aperture and permits fabrication of single mode waveguides that match single-mode fibers in both cross sectional dimensions and numeric aperture. When the core and cladding materials are comprised of two or more miscible monomers, the index at each layer of a waveguide can be precisely tailored by mixing selected pairs of high index and low index monomers. This property can be used to precisely control the mode of a waveguide and can be used to fabricate large-size single-mode waveguides that match commercial single-mode fibers in both cross sectional dimensions and numeric aperture.
In this invention, a planar waveguide structure is formed in which only a buffer, a core, and an overclad are applied to a substrate. A buffer layer is formed on a substrate and cured. A core layer is applied on top of the buffer layer. During the core application and cure, diffusion of low molecular weight, high index of refraction material takes place and increases the index of the buffer. A gradient index is formed through the buffer. The gradient index then sharply falls off with distance into the buffer region. However, the core, cladding and buffer are chosen such that optical multimode behavior is frustrated for all potential values of buffer index. In addition, an overcladding is applied, which coats both the sides and the top of the core. A similar diffusion of high index monomer occurs thereby assuring a gradient index around the core. According to this invention, clear single-mode performance can be combined with exceptionally low coupling loss due to the improved mode matching with round-core single-mode fibers.
The components described in this invention are formed by only three layers on top of the substrate: a buffer layer of index nb; a core layer of index nc; an overclad layer of index no with nb less than no less than nc. In this invention, multi-layer photonic integrated circuit components are fabricated with a core of index nc surrounded on top by an overclad of index no less than nc and at the bottom by a buffer of index nb less than no, permitting one to achieve low optical loss, low polarization dependent loss, no optical power drop with heating, no secondary waveguide under the core waveguide; no secondary reflections and no (or very low) cladding mode loss in the presence of a volume grating.